The liver and regulating blood nutrients

The big idea: All the blood leaving your gut passes through the liver before reaching the rest of the body.

This makes the liver the body's chemical hub: it adjusts the nutrients in the blood, stores some, removes others, and breaks down toxins.

The cells that do this work are called hepatocytes (liver cells). A key job is keeping the level of glucose in the blood steady — neither too high nor too low.

Liver

A large organ that processes nutrients and toxins in the blood coming from the gut; the body's main 'chemical factory'.

Hepatocyte

A liver cell — the cell type that carries out the liver's many chemical jobs, including regulating blood nutrients.

Glycogen

A storage carbohydrate (a polymer of glucose) that the liver makes when blood glucose is high and breaks down when it is low.

Blood glucose regulation

Keeping the concentration of glucose in the blood close to a steady set point, mainly by storing or releasing glucose in the liver.

Bile

A fluid made by the liver; it carries excess cholesterol and the waste pigment bilirubin out of the body in the faeces.

Why the liver sits where it does: Blood from the gut, loaded with newly absorbed nutrients, reaches the liver first.

That lets the liver adjust the nutrient levels before the blood reaches every other organ — so the rest of the body receives blood with a steady, safe composition.

Your blood glucose changes throughout the day — it rises after a meal and falls between meals.

The liver smooths these swings out. It is the effector in a negative-feedback loop: whichever way glucose drifts, the liver's response pushes it back toward normal.

Negative feedback

A control system in which a change in a level triggers a response that OPPOSES the change, returning the level toward its set point.

Insulin

A hormone (from the pancreas) released when blood glucose is HIGH; it makes the liver take up glucose and store it as glycogen, lowering blood glucose.

Glucagon

A hormone (from the pancreas) released when blood glucose is LOW; it makes the liver break glycogen back into glucose, raising blood glucose.

When blood glucose is HIGH (after a meal): After eating, glucose floods into the blood and the level rises.

The pancreas releases insulin. In response, hepatocytes take up glucose and join the molecules together into glycogen for storage.

Removing glucose from the blood makes the level fall back toward normal — the response opposes the rise.

When blood glucose is LOW (between meals): Between meals, the blood glucose level falls.

The pancreas releases glucagon. In response, hepatocytes break glycogen back down into glucose and release it into the blood.

Adding glucose makes the level rise back toward normal — again the response opposes the change.

A memory hook: Insulin → IN (glucose goes into store as glycogen, lowering blood glucose).

Glucagon → Glucose comes back (glycogen is broken down, raising blood glucose).

Both are negative feedback: the liver's response always opposes the change.

How this is tested: On Paper 3 a 2-mark Outline asks how hepatocytes regulate nutrient levels in the blood — the expected answer is the store-as-glycogen / release-from-glycogen glucose loop.

A related 2-mark Outline asks how the body removes excess cholesterol — the liver puts it into bile, which is lost in the faeces.

Data-handling versions show a blood-glucose-versus-time trend after a meal and expect you to reason about insulin, glycogen storage and negative feedback from the shape of the curve.

✓ Why this scores full marks: The two marks come from the two opposite responses — store glucose as glycogen, and break glycogen back to glucose.

Writing only one direction (e.g. just 'stores glucose') caps you at 1 mark. A good answer always shows both halves of the negative-feedback loop.